With the advancement in technology, mobile telecommunications have evolved from the Advanced Mobile Phone System (AMPS) in the 1980's to the second generation (2G) in the 1990's, including those based on the GSM standard, and on to the third generation (3G) in the present day. One of the 3G standards is TD-SCDMA, which uses time-division duplexing (TDD). By dynamically adjusting the number of time slots used for downlink and uplink, a TD-SCDMA mobile telecommunication network can more easily accommodate asymmetric traffic with different data rate requirements on downlink and uplink than a network based on frequency-division duplexing (FDD).
In various markets, mobile telecommunication networks based on 3G standards are being deployed in place of or in addition to existing 2G networks. As such, multimode operation is a critical success factor as well as necessity for 3G networks. Without the capability of multimode operation, mobile handset users may lose coverage when entering areas where a 3G network has yet to be deployed. In markets where networks based on the TD-SCDMA standard are deployed, multimode operation in mobile handsets allow users to enjoy high-speed data applications with 3G TD-SCDMA coverage while having the assurance of GSM cellular coverage in areas where 3G TD-SCDMA coverage is not yet available.
For TD-SCDMA single-mode operation, a mobile handset needs two frequency synthesizers in the radio, typically with one frequency synthesizer designated for the transmission and receiving of TD-SCDMA signals and the other designated for the reception of TD-SCDMA signals from neighbor cells. The frequency synthesizers are typically in the form of phase-locked loops (PLL). However, in order to provide TD-SCDMA/GSM multimode operation, the mobile handset needs an additional frequency synthesizer for GSM monitoring because no suitable monitoring gaps for GSM have been defined that will guarantee the detection of GSM neighbor cells in all possible TD-SCDMA configurations for data transmission and neighbor cell monitoring. As a result, existing multimode handsets require two independent receiver paths and three frequency synthesizers.
In general, there are four types of activities in TD-SCDMA/GSM multimode operation. Firstly, there are TD-SCDMA data receiving activities during those time slots used for receiving. According to the TD-SCDMA standard, the time slots TS0 and TS6 may only be used for receiving. Secondly, there are TD-SCDMA signal reception activities that take place during the time slot TS0, such as inter-frequency monitoring for example. Thirdly, there are TD-SCDMA transmission activities during those time slots used for transmission. According to the TD-SCDMA standard, the time slot TS1 may only be configured for transmission, and time slots TS2 TS3 TS4 TS5 may be configured for transmitting or receiving. Fourthly, there are GSM receiving or monitoring activities to receive signals from neighbor cells. GSM monitoring should not overlap in time with TD-SCDMA transmission time slots because TD-SCDMA transmission tends to de-sense signal reception for GSM monitoring.
FIG. 1 illustrates a timing diagram of TD-SCDMA/GSM multimode operation using three frequency synthesizers, under a worst-case scenario in which High-Speed Uplink Packet Access (HSUPA) protocol category 5/6 2.2 Mbps uplink is occurring under a conventional scheme. A first frequency, frequency 1, is used for TD-SCDMA transmission and receiving activities. A second frequency, frequency 2, is used for TD-SCDMA signal reception, such as, for example, inter-frequency monitoring for signals from neighbor cells. A third frequency, frequency 3, is used for GSM monitoring for signals from neighbor cells.
Here, in the worst-case scenario with respect to GSM monitoring, the TD-SCDMA time slots UpPTS and TS1 through TS5 are all used for transmission on frequency 1. Time slot TS6 is used for signal reception on frequency 1. Time slot TS0 is used for signal reception on frequency 2, such as for TD-SCDMA inter-frequency monitoring for signals from neighbor cells. Throughout the entire time, or continuously, GSM monitoring for signals from neighbor cells is performed on frequency 3. As shown in FIG. 1, in the conventional scheme, two frequency synthesizers, or two PLLs, are required for activities based on the TD-SCDMA standard. Without a third PLL, either GSM monitoring or TD-SCDMA signal reception cannot be carried out. The third PLL is therefore required for TD-SCDMA/GSM multimode operation.
Moreover, in order to avoid de-sensing signal reception in the GSM mode due to TD-SCDMA transmission, GSM receiving (i.e., monitoring) and TD-SCDMA transmission cannot overlap in time. However, this is particularly challenging since in some configurations most of the time in a TD-SCDMA subframe is used for TD-SCDMA transmission, which leaves very little time available for GSM monitoring.
An existing solution to the aforementioned issues utilizes two separate radios in a given mobile handset, one for TD-SCDMA and the other for GSM. Still, as the integration level is low, such solution tends to be expensive, and three frequency synthesizers are required. Even if the two radios are combined, three frequency synthesizers are still required. This makes the design of an integrated-circuit multimode radio more complicated, risky to layout, and expensive. In any case, the need to avoid overlap between GSM receiving and TD-SCDMA transmission remains a difficult issue.
Additionally, in some TD-SCDMA inter-frequency baton handover scenarios there may be adjacent receiving and transmission time slots, resulting in change from transmission in one frequency to receiving in another frequency in zero time. When frequency synthesizers are shared, an issue arises as there is no time to re-lock a shared frequency synthesizer. Moreover, during multimedia broadcast and multicast services (MBMS), there is zero time to move from receiving in one frequency in TD-SCDMA time slot TS6 to receiving in another frequency in TD-SCDMA time slot TS0. As two separate frequency synthesizers are needed for this TD-SCDMA operation, this is an issue when the frequency synthesizers are shared.